Unit I - Plant Reproduction, Breeding, and Hybrid Development | Commercial Plant Breeding & Seed Industry

Unit I

Commercial Plant Breeding & Seed Industry

Contents
Types of crops and modes of plant reproduction
Line development and maintenance breeding in self and cross-pollinated crops (A/B/R and two-line system) for the development of hybrids and seed production.

Types of crops

In Commercial Plant breeding, crops are generally categorised Based on Reproductive Characteristics: 

• Self-Pollinated Crops: In these crops, the transfer of pollen from the male reproductive organ (anther) to the female reproductive organ (stigma) occurs within the same flower or between flowers of the same plant. Self-pollination leads to a high degree of genetic uniformity and stability in the offspring. Examples of self-pollinated crops include wheat, rice, barley, and soybean.
• Cross-Pollinated Crops: These crops require the transfer of pollen from the anthers of one plant to the stigma of a different plant of the same species for fertilization. Cross-pollination promotes genetic diversity and the potential for heterosis (hybrid vigour) in the offspring. Examples of cross-pollinated crops include maize, cotton, and sunflower.
• Vegetatively Propagated Crops: Some crops can be propagated without involving seeds, using vegetative parts like stems, leaves, or roots. This method results in genetically identical offspring, providing consistency in traits. Examples of vegetatively propagated crops include potatoes, bananas, and sugarcane.
• Hybrid Crops: Hybrid crops are produced by crossing two genetically diverse parent lines to achieve superior performance in the offspring. The first-generation hybrid (F1) shows enhanced traits like yield, disease resistance, and uniformity. Examples of hybrid crops include hybrid maize, hybrid tomato, and hybrid sunflower.

Crops can be further categorised:

Based on Usage:
Food Crops: Crops grown primarily for human consumption, providing nutrients and energy.
Cash Crops: Crops grown for sale in the market to generate income.
Industrial Crops: Crops used as raw materials in industries (e.g., rubber, jute).
Horticultural Crops: Fruits, vegetables, flowers, and ornamental plants.
Fiber Crops: Crops grown for fibers used in textiles (e.g., cotton, flax).

Based on Season:

Kharif Crops: Kharif is the monsoon season, typically spanning from June to October. This period brings abundant rainfall, providing vital moisture to the soil. Kharif crops include staple foods like rice and maize, as well as soybeans, cotton, and certain oilseeds.

Rabi Crops: Rabi follows the Kharif season, lasting from October to March. This season coincides with the winter months when the climate is cooler and drier. Wheat, barley, pulses, and mustard are typical Rabi crops.

Zaid Crops: Zaid is a shorter season, occurring between Rabi and Kharif. It spans from March to June. Zaid crops include cucumbers, watermelons, and certain leafy vegetables. The Zaid season complements the other two seasons, providing a window for growing crops that thrive in the heat.

Modes of Plant Reproduction

• Sexual Reproduction: In sexual reproduction, plants produce seeds through the fusion of male and female gametes. Pollen, containing male gametes, is transferred to the female reproductive structures of the flower, leading to fertilization and seed formation. Sexual reproduction contributes to genetic diversity and adaptation to changing environments.
• Asexual Reproduction: Asexual reproduction involves the production of new plants from vegetative parts without involving seeds. Common methods of asexual reproduction include cuttings, runners, rhizomes, and tubers. This process results in genetically identical offspring, ensuring the preservation of desirable traits.
• Apomixis: Apomixis is a form of asexual reproduction in which seeds are formed without the process of fertilization. The seeds produced through apomixis are genetically identical to the parent plant, maintaining the genetic traits without any recombination.

Here are some common modes of plant reproduction within these categories:

Sexual Reproduction:

1. Pollination:

- Pollination is the transfer of pollen from the male reproductive organ (anther) to the female reproductive organ (stigma) of a flower.

- It can be facilitated by wind, water, or animals (insect pollination).

2. Fertilization:

- After pollination, pollen grains germinate on the stigma and grow down the style to reach the ovary, where fertilization occurs.

- Fertilization results in the formation of a zygote, which develops into an embryo within a seed.

3. Seed Formation:

- The fertilized ovule develops into a seed containing the embryo, endosperm (in some cases), and protective seed coat.

- Seeds are dispersed and can germinate to produce new plants under favorable conditions.

Asexual Reproduction:

1. Vegetative Propagation:

- Vegetative propagation involves the production of new plants from vegetative parts of the parent plant, such as stems, leaves, and roots.

- Examples include cuttings, runners, and rhizomes.

2. Bulb Division:

- Bulb plants like onions and lilies reproduce by dividing the bulbs into smaller sections, each of which can grow into a new plant.

3. Tubers and Corms:

- Plants like potatoes produce tubers that can be planted to grow into new plants.

- Corms are similar structures found in plants like crocuses and gladioli.

4. Layering:

- Layering involves bending a branch of a plant to the ground and allowing it to root while still attached to the parent plant.

- Once rooted, the branch can be separated and grown as an independent plant.

5. Grafting and Budding:

- Grafting involves joining parts of two different plants (scion and rootstock) to create a new plant with desired characteristics.

- Budding is a type of grafting where a bud is attached to a rootstock.

Each mode of reproduction has its advantages and serves specific purposes in plant propagation and evolution. Sexual reproduction contributes to genetic diversity, while asexual reproduction ensures the rapid propagation of genetically identical plants with desirable traits.

Line Development and Maintenance Breeding in Self and Cross-Pollinated Crops for Hybrid Development and Seed Production:

Line Development (A/B/R System):

• A-Line (Androecious Line): A-lines are male-sterile lines (female parent line) that lack functional anthers. Emasculation is a critical step in the development of the A-line. They cannot self-pollinate and rely on pollen from B-lines for fertilization.
• B-Line (Bisexual Line): It serves as the male parent line. The B-line is bred to be highly male-fertile, meaning it produces abundant and viable pollen. This pollen is used to fertilize the emasculated flowers of the A-line.
• R-Line (Restorer Line): The R-line carries specific genes that restore fertility to the hybrid plants resulting from the cross between the A-line and the B-line.
• These hybrid plants are initially sterile due to genetic factors from the B-line. The R-line's genes overcome this sterility, allowing the hybrid plants to produce seeds.
• When crossed with A-lines, they ensure seed set and yield in hybrid offspring.

Because the A and B lines are isogenic to each other, the cross between them does not change the genotype and characteristics of the A-line.

Hybrid Development:

The controlled cross-pollination between the emasculated A-line and the male-fertile B-line results in the production of F1 hybrid plants.The F1 hybrids inherit traits from both parents and often display hybrid vigor, which leads to improved yield and other desirable characteristics.

Restoration of Fertility:

The F1 hybrid plants, although vigorous, are initially sterile due to the influence of the B-line's male-sterile genes.To restore fertility, the R-line is used to introduce restorer genes that overcome this sterility, allowing the hybrids to produce viable seeds.

Seed Production:

Once fertility is restored in the F1 hybrid plants, they can self-pollinate and produce it seeds.These seeds carry the combined traits of the A-line and B-line, as well as the benefits of hybrid vigor.

Commercial Seed Production:

The seeds produced by the F1 hybrids are used for commercial seed production. These seeds will yield plants with the desired traits in subsequent generations.

Trait Improvement:

Through multiple generations of selection and controlled crosses, breeders can continually refine and improve the traits of the A-line, B-line, and R-line to enhance the quality of the resulting hybrids and seeds.

Line Maintenance:

• Maintaining Genetic Purity: To avoid contamination and maintain purity, careful isolation is necessary between different lines during seed multiplication.
• Rogueing: Any off-type or undesirable plants are removed during the growing season to prevent contamination and ensure the purity of the lines.
• Hand Emasculation and Controlled Pollination: To produce pure seeds, emasculation of A-lines is done manually to prevent self-pollination. Controlled pollination with pollen from the corresponding B-line is then carried out.

Line Development (Two-Line System):

• A-Line (Female Parent): The A-line, also known as the female parent, is bred to have a cytoplasmic male sterility (CMS) trait. This CMS trait prevents the A-line from producing functional pollen, rendering it incapable of self-pollination. The A-line's primary role is to serve as the female parent in hybrid seed production.
• R-Line (Restorer Line, Male Parent): R-lines are male lines with superior pollen production and fertility. They ensure proper pollination and fertilization of A-lines. The R-line carries specific genes that restore male fertility in the hybrid plants. This gene counteracts the effects of the CMS trait in the A-line, allowing the R-line to produce functional pollen.

Sterility Mechanism: The A/R line system relies on a sterility mechanism present in the F1 hybrid plants. The F1 hybrids produced by crossing the A-line and the R-line are initially male-sterile, meaning they cannot produce viable pollen.

Restorer Genes: The R-line possesses restorer genes that overcome the sterility in F1 hybrids.

Hybrid Vigor: The F1 hybrids benefit from hybrid vigor (heterosis), where they exhibit improved growth, yield, and other traits compared to their parent lines. This vigor is a result of the combination of genes from both parent lines.

Seed Production:The fertile F1 hybrids, now capable of producing pollen, can be used to pollinate other plants of the A-line. This controlled pollination between the F1 hybrid and the A-line results in seed production.

Commercial Seed Distribution: The seeds obtained from the cross between the F1 hybrid and the A-line are collected and distributed for commercial seed production.

Stability and Continual Improvement: Line maintenance involves ensuring the stability of both the A-line and R-line populations over time. Breeders continually evaluate the performance of hybrids and parent lines to select the best individuals for future crosses.

Adaptation and Customization: The A/R line system can be adapted to various crop species and specific trait requirements. It allows breeders to create hybrids tailored to different environmental conditions and market demands.

Line Maintenance:

• Maintaining Genetic Purity: Like in self-pollinated crops, maintaining genetic purity is crucial in the two-line system to avoid contamination.
• Isolation and Controlled Pollination: To produce hybrid seeds, A-lines and R-lines are isolated from each other to prevent unwanted cross-pollination. Controlled pollination is carried out to ensure fertilization of A-lines by the pollen from R-lines.

Line Development and Maintenance Breeding in Self-Pollinated Crops:

1. Self-Pollinated Crops: Self-pollinated crops have flowers that are typically equipped to pollinate themselves, meaning pollen from the same flower or another flower on the same plant can fertilize the ovules. Examples of self-pollinated crops include wheat, rice, and beans.

2. Line Development: In self-pollinated crops, the concept of male sterility isn't as relevant as in cross-pollinated crops. However, maintaining genetic purity is crucial to achieve desirable traits consistently. Breeders select and maintain pure lines through generations of self-pollination. These pure lines are then used as parents to create hybrid varieties.

3. Hybrid Development: To develop hybrids in self-pollinated crops, breeders follow these steps:

• Select two or more pure lines with complementary desirable traits.
• Crossbreed the selected lines to produce an F1 hybrid generation.
• The F1 hybrid exhibits hybrid vigor or heterosis, resulting in improved traits compared to the parents.
• Hybrid seeds from the F1 generation are produced and distributed to farmers.

4. Seed Production: Producing hybrid seeds in self-pollinated crops can be challenging due to the potential for self-pollination. Breeders often develop mechanisms to ensure outcrossing between different F1 plants to maintain hybrid vigor. Techniques like emasculation (removal of male reproductive organs) and controlled pollination are used to prevent self-pollination and maintain genetic diversity.

Line Development and Maintenance Breeding in Cross-Pollinated Crops:

1. Cross-Pollinated Crops: Cross-pollinated crops require pollen transfer between flowers of different plants for fertilization. Examples include maize, sunflower, and many fruit trees.

2. Line Development: In cross-pollinated crops, creating and maintaining pure lines is vital for successful hybrid development. Breeders select and maintain pure lines through careful isolation, controlled pollination, and seed collection from individual plants.

3. Hybrid Development: For cross-pollinated crops, hybrid development often involves the A/B/R or two-line system:

• A line (female parent) is male-sterile.
• B or restorer line (male parent) restores fertility to the A line.
• Hybrid seeds are produced by crossing the male-sterile A line with the restorer line.

4. Seed Production: Producing hybrid seeds in cross-pollinated crops requires maintaining genetic purity of the parental lines and ensuring proper cross-pollination. Isolation distances, isolation techniques, and pollination management are critical to avoid contamination and achieve consistent hybrid seed production.

Line development and maintenance breeding play crucial roles in the development of hybrid varieties and the production of pure, genetically stable seeds. In self-pollinated crops, the A/B/R system is employed, while the two-line system is used in cross-pollinated crops. These breeding strategies are essential in commercial plant breeding and seed production to ensure superior performance, genetic purity, and sustainable agriculture practices.

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